Myocardial bridging is a congenital coronary abnormality in which a coronary segment runs through the myocardium intramurally (e.g., a segment of a coronary artery tunnels through the myocardium instead of lying on top of it), resulting in systolic compression of the tunneled segment. The frequency of myocardial bridging has been reported to be 1.5% to 16% in coronary angiography and as high as 80% in autopsy series. Myocardial bridging can cause cardiac-related complications such as ischemia and acute coronary syndromes, and coronary spasms.
Coronary artery disease, in turn, may cause the blood vessels providing blood to the heart to develop lesions, such as a stenosis (abnormal narrowing of a blood vessel). As a result, blood flow to the heart may be restricted. A patient suffering from coronary artery disease may experience chest pain, referred to as chronic stable angina during physical exertion or unstable angina when the patient is at rest. A more severe manifestation of disease may lead to myocardial infarction, or heart attack.
Myocardial bridging may occur partially or completely. For example, a segment of a coronary artery of a patient may be completely surrounded by the patient's myocardium (e.g., 100% tunneling of the vessel into the myocardium). Alternatively, the abnormality may occur as partial myocardial bridging—e.g., 30%-99% of the circumference of a segment of a coronary artery of the patient is surrounded by the myocardium, with tapering and/or reduction of cross-sectional area of the coronary artery.
Typically, myocardial bridging may be diagnosed by coronary angiography or intravascular ultrasound imaging (IVUS) based on one or more of several features, including significant percent lumen diameter narrowing, persistent diastolic diameter reduction, a “milking effect” in angiography, and/or a “half moon” phenomenon in IVUS. Besides morphological evaluation, intracoronary Doppler may show increased flow velocity, retrograde systolic flow, and reduced coronary flow reserve in myocardial bridging. The functional significance of myocardial bridging may be evaluated using fractional flow reserve (FFR) with the use of inotropic agents. FFR may be defined as the ratio of the mean blood pressure and/or flow downstream of a location, such as a lesion or location of myocardial bridging, divided by the mean blood pressure and/or flow upstream from the location, under conditions of increased coronary blood flow, e.g., when induced by intravenous administration of adenosine.
However, traditionally, these methods are invasive procedures and may involve the use of inotropic agents such as dobutamine to induce maximal myocardial contraction. In some cases, diastolic FFR may be more relevant than the conventional FFR to the evaluation of myocardial bridging due to overshooting of systolic pressure, which may lead to underestimation of severity when assessed by the conventional FFR.
As these physiologic and hemodynamic conditions of myocardial bridging may hamper the use of conventional invasive FFR, it would be useful to differentiate patients with fixed stenosis from those with myocardial bridging for an accurate blood flow simulation in assessing the hemodynamic significance of lesions.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure.